Production of tellurium



1941- w. G. WOLL EI'AL 2,258,963

PRODUCTION OF TELLURIUM Filed July 20, 1939 ATTORNEYS Patented Get. i4, 194i PRODUCTION or TELLURIUM Wesley G. Woll, Tottenville, N. Y., and Robert T.

Gore, lilrexel Hill, Pa, assignors to International Smelting & Refining Company, New York, N. Y a corporation of Montana Application July 20, 1939, Serial No. 285,500

Claims.

This invention relates to the production of tellurium, and has for its object the provision of certain improvements in the electrolytic production of metallic tellurium.

The invention is based upon our discovery that metallic tellurium of excellent quality can be electro-deposited in reguline state as a firm adherent deposit upon a suitable cathode from a caustic alkali electrolyte containing an alkalimetal tellurite. The reactions involved are generally as follows: A. Tellurium dioxide (TeOz) is readily soluble in an aqueous solution of an alkali-metal hydroxide, e. g. caustic soda (NaOH) or caustic potash (KOH); Tellurium dioxide dissolved in an aqueous solution of caustic soda forms sodium tellurite (NazTeOs) in aqueous solution according to the following reaction:

TeOz+2NaOH=Na2TeOa+Hz0 B. The passage of an electric current through gives the following (by adding Equations 1, s, 4. 5 and 6 and subtracting Equation 2) 7 (7) Na TeO H O+ 4 faradays= Te 0 2NaOH As indicated in the foregoing equation, the sodium hydroxide is regenerated in the solution, and hence the consumption of alkali-metal hydroxide in the electrolytic process is theoretically zero.

While the ionization of sodium tellurite into sodium ions and tellurite ions is considerable, the secondary ionization of the tellurite ions to give tellurium ions is small, and hence the concentration of tellurium ions in the solution at any time will be only a small fraction of the total Y tellurium concentration of the electrolyte.

the sodium tellurite solution, under the conditions characteristic of the invention as hereinafter described, reduces the sodium tellurite with the electrodeposition of metallic tellurium on the cathode. The reactions for this reduction are as follows: (1) NazTeOsZZNafi-l-TeOF (2) 2NaOHZ2Na++2OH- The foregoing are the primary ionizations that occur in the electrolyte, but inasmuch as metallic tellurium is obtained on the cathode, a secondary ionization of the tellurite ions must occur, giving free tellurium ions in solution. Since water ionizes to a slight degree,

this secondary ionization can be illustrated by this reaction: 4) Teos=+3H+ Te+++++3oH and the cathode reaction would then be (5) v +++++4e 4 and the anodr'a-reaction In the practice of the invention, the electrolyte is an aqueous solution of an alkali-metal tellurite, preferably sodium tellurite. The electrolyte is made up by first dissolving the required amount of caustic soda in water and then dissolving, in the resulting solution, the required amount of tellurium dioxide. We have used with excellent results an electrolyte made up by first dissolving 104 grams of caustic soda in about 750 c. c. of water. In this solution 188 grams of tellurium dioxide are dissolved, and the solution is then diluted with water to one liter. The resulting solution has a tellurium concentration of about -150 grams per liter, and about 10 grams per liter of free caustic soda. These concentrations are notcritical. Satisfactory deposition of metallic tellurium on the cathode can be obtained so long as the tellurium concentration is not allowed to fall much below 50 grams per liter. Satisfactory deposition of metallic tellurium in reguline state as a firm adherent deposit can be obtained from a solution of any concentration above 50 grams per liter up to the saturation point of sodium or potassium tellurite in water.

Caustic potash is just as satisfactory as caustic soda in the practice of the invention. The only reason for preferring caustic soda is because it'is less expensive. Tellurium dioxide is notsoluble in any other common alkalinesolutions except ammonium hydrate solutions, and here the solubility is so low that a sufliciently high concentration of tellurium to allow economic operation of the electrolytic process'can'fiot be obtained. Lithium hydroxide, rubidium hydroxide, and caesium hydroxide are not considered common alkalies and the cost of using them as a solvent for tellurium dioxide would be prohibitive. The

the anode tellurites of the alkaliearth metals such. as calcium, barium, and strontium are insoluble salts, and hence alkaline compounds of these elements cannot be employed.

As hereinbefore stated, the caustic soda is regenerated in the course of the electrolysis, and the spent electrolyte can be brought back-to its original concentration of tellurium by dissolving therein an amount of tellurium oxide equivalent to the amount of tellurium electrodeposited on the cathode. Small additions of caustic soda are needed from time to time to make up the small losses thereof in the caustic soda consumed in precipitating impurities and also due to the small amount of caustic soda carried off by the escaping oxygen into the ventilating system.

The metallic tellurium is preferably deposited on cathodes of stainless steel. No grease or other agent need be used tomake the deposit strip readily. The hard, brittle tellurium falls away easily from the smooth cathode surface. Such a cathode may be stripped in not more than two minutes. Tellurium deposits just as readily on iron or lead as on stainless steel. However, cathodes of iron or lead cannot be so readily stripped as stainless steel cathodes, due to the firm bondformed between'the metallic tellurium and iron or lead. It is likely that this firm bond is due to the formation of a layer of iron'telluride or lead telluride. No reaction between the deposited tellurium and stainless steel cathode seems to occur.

Insoluble anodes of iron or the like are used in the practice of the invention. The anodes are preferably smaller than the cathodes to eliminate building up of the deposit on the cathode edges, and hence short circuits in the tanks. The current density should be from 10 to 20 amperes per square foot of cathode surface but may be from 1 to amperes per square foot. Above 20 amperes per square foot the deposit becomes soft, spongy and non-adherent and because of this condition is not easily washed. Hence the quality of the metal may be lowered by occlusion of impurities. The electrolyte should preferably be heated to a temperature slightly in excess of 100 F. Under optimum operating conditions no external heat need. be applied to the electrolyte, since the passage of the electric current will maintain the electrolyte at the desired temperature.

The single figure of the accompanying drawing diagrammatically illustrates an apparatus for the practice of the invention.

The apparatus shown in the drawing comprises two similar electrolytic tanks 5. Each tank has an anode bus-bar 6 along one side thereof and a cathode bus-bar I along the other side thereof. The cathode bar 'I of one tank is electrically connected to the anode bar 6 of the other tank by a conducting plate 8, and the other anode and cathode bars of the tanks are connected to an appropriate source of direct current 9.

Anodes I0 alternate with cathodes il throughout the length of each tank. Each anode I0 consists of a pair of sheet iron strips appropriately fastened to a copper'bar I3. One end of the bars l2, in each tank, rests on and electrically contacts the anode bar 6 while the other end of each bar it rests upon an insulated support on the oppos'ite side of the tank. Each cathode H consists of a pair of stainless steel strips fastened to a copper bar IS. The cathodes are in two narrow strips rather than one large sheet in order to facilitate the stripping of the deposited tellurium. One end of each bar IS, in each tank, rests on and electrically contacts'the cathode bar I while 1;; inches spaced inch apart. There are the other end of each bar It rests upon an insulated support on the opposite side of the tank.

Fresh or pregnant electrolyte is stored in a feed tank l4. The tank I4 is provided with two outlet pipes I5 near the bottom thereof. Each pipe [5 has a cock It for controlling the delivery of electrolyte to funnels ll. From the funnels II the electrolyte is delivered by pipes Hi to each of the tanks 5 near one end thereof.

Spent or barren electrolyte is discharged from the opposite end of the tanks 5 to a tank l9 through a; common pipe line 20. A hood 2| is mounted above the tops of the tanks 5. Gases evolved during the electrolytic process are drawn from the hood through a communicating pipe 22 by a suction fan 23 and appropriately disposed of.

The spent electrolyte tank [8 is preferably of sufficient capacity-to hold all of the electrolyte flowing through the two tanks 5 in 24 hours. Preferably, two such tanks are provided. When one tank I! is full at the end of thetwenty-fourhour period, the electrolyte is permitted to flow into the other tank for the succeeding twentyfour-hour period. Suflicient tellurium dioxide is dissolved in the full tank to bring the electrolyte back to its original concentration of tellurium. Theresulting solution is allowed to-"settle over night in the tank so that the precipitated impurities (flocculent hydroxides) will separate from the supernatant clear solution. The clear superlocated one inchand seven inches, respectively,-

above thebottom of the tank. The pipes 24 and 25 are provided with cocks 26 and 21 respectively. The clear supernatant regenerated electrolyte is first drawn oil. from the tank l9 through the upper pipe 25. The sedimented liquor is then drawn off through the lower pipe 24. The liquor drawn off through both pipes is delivered to a filter consisting of a wooden frame covered with muslin. The filtered regenerated electrolyte is stored in an appropriate tank, from which it can be pumped, for ample, by a steam syphon, as required, to the c ctrolyte feed tank l4. Ex-

cepting the delivery of the filtered regenerated electrolyte to the tank It, all other flow of electrolyte through the apparatus is by gravity.

The following example is cited merely to illustrate the practice of the invention and is nowhere restrictive of the invention. Each of the electrolytic tanks 5 has a capacity of liters with dimensions 42 x 13 x 15% inches. The capacity of the feed tank It is 600 liters with dimensions 60 x 1'7 x 36 inches. Each of the two spent electrolyte tanks l8 has a capacity of 600 liters with dimensions 44 x 30 x 28 inches.

The anodes I 0 are of 1; inch sheet iron in pairs of strips 15 x 4 inches spaced 1% inches apart. The anodes dip into the electrolyte 13% inches. The anodes are fastened to copper bars 17% x i l 1: V inches by iron machine screws. There are 19 anodes for each tank, giving an anode area of 31.5 square 'feet. The current flow through each tank.is 430 amperesfgiving a current density of 13.6 amperes per square foot.

The cathodes are of inch 18-8 stainless steel escapes one inch. All tanks, pipe lines and cocks are of iron except the cocks l5-for regulating the flow of electrolyte into the electrolytic tanks and these are preferably of pyrex glass. Each of the tanks may advantageously be constructed of inch sheet iron. The pipes l5 may advantageouslybe A inch pipes two inches above the bottom of the tank It. The pipes 24 and 25 may similarly be $41 inch.

" The current efficiency is always approximately 99+% and the yield is in the range of 1.25 to 1.50 pounds of metallic tellurium per kwh. The voltage per tank ranges from 2.0 to 2.4. Normal operating voltage is about 2.1. No external heat is applied to the electrolyte since the passage of the current keeps the temperature in the electrolytic tanks slightly above 100- F. with a normal room temperature. This is desirable since no frothing of the electrolyte due to the evolution of the oxygen occurs above 90 F. When starting up with cold electrolyte, some frothing may occur until the temperature rises over 90 F., particu- 1. The improvement in the electrolytic production of metallic tellurium which comprises electrodepositing metallic tellurium in reguline state as a firm adherent deposit upon a suitable cathode at a current density of from 10 to 20 amperes per square foot of cathode surface from a caustic alkali electrolyte containing not less than about 50 grams per liter of tellurium as an alkali-metal tellurite.

2. The improvement in the electrolytic production of metallic tellurium which comprises electrodepositing metallietellurium in reguline state as a firm adherent deposit upon a suitable cathode at a current density of not less than 10 amperes per square foot of cathode surface from a caustic alkali electrolyte containing upwards of 50 grams per liter of tellurium as an alkali-metal tellurite.

3. The improvement in the electrolytic production of metallic tellurium which comprises electrodepositing metallic tellurium in reguline state as a firm adherent deposit upon a stainless steel cathode in an electrolytic cell having an insoluble anode and a caustic alkali electrolyte containing not less than about 50 grams per liter of tellurium as sodium or'potassium tellurite with a current density of from 10 to 20 amperes per square foot of cathode surface, continuously supplying fresh electrolyte to the cell and simultaneously withdrawing from the cell an equivalent volume of spent electrolyte, and regenerating the spent electrolyte by the solution thereinof tellurium oxide larly if the solution is not already saturated with dissolved oxygen.

The invention permits the production of metallic tellurium of excellent quality at high current efflciency and low cost. The advantages of the invention may be further briefly summarized as follows:

(1) The large percentage of solvent recovered during electrolysis.

(2) The minimum of toxic vapors developed during operation, making the process considerably safer from a health standpoint than me reduction methods.

(3) The excellent quality of metal obtained.

The production obtained has exceptional purity,

analyzing in the neighborhood of 99.9% tellurium, and containing but very small amounts of sodium (from .006% to .039%), selenium (from trace to .05%) and iron (from .002% to .02%).

(4) High current emciency. '(5) Ease of operation and low cost. I We claim:

to form fresh electrolyte.

4. The improvement in the electrolytic production of metallic tellurium which comprises electrodepositing metallic tellurium in reguline state as a firm adherent deposit upon a suitable cathode at a current density of from 10 to 20 amperes per square foot of cathode surface from a caustic alkalielectrolyte containing upwards of grams per liter of tellurium as an alkali-metal te1lurlte.. and regenerating the resulting spent electrolyte for return to the process by dissolving tellurium oxide therein.

5. The improvement in the electrolytic, production of metallic telluriumwhich comprises electrodepositing metallic tellurium'in reguline state as a firm adherent deposit upon a suitable cathode at a current density of about 12.7 amperes per square foot of cathode surface from a caustic alkali electrolyte containing not less than about 50 grams per liter of tellurium as an alkali-metal tellurita,

I WESLEY G. WOLL.

ROBERT T. GORE. 

